Problems to be noted in the design of gas-assisted injection molding products

Gas-assisted injection molding technology is an emerging plastic injection molding technology. Its principle is to use high-pressure gas to create a hollow section inside the plastic part, and use gas pressure-holding instead of plastic injection pressure-holding to eliminate the product shrinkage and complete the injection molding process. The process of gas-assisted injection molding mainly consists of three stages: plastic melt injection, gas injection, and gas holding pressure.

In the short injection method, the gas first pushes the melt to fill the cavity and then holds the pressure; in the full injection method, the gas-only plays the role of holding pressure.
 

The advantages of gas-assisted injection molding technology mainly include

 1. Solve the problem of surface shrinkage of the parts, which can greatly improve the surface quality of the parts.
 2. Local gas channel thickening can increase the strength and dimensional stability of the part, and reduce the internal stress of the product and warpage deformation.
 3. Save raw materials, up to 40%-50%.
 4. Simplify product and mold design, reducing the difficulty of mold processing.
 5. Reduces cavity pressure, reduces clamping force, and extends mold life.
 6. Faster cooling and shorter production cycle.

Gas-assisted injection molding technology has incomparable advantages compared with the ordinary injection molding process and is regarded as a revolution of the injection molding process, which is widely used in almost all fields of plastic parts such as home appliances, automobiles, furniture, and daily necessities. In the field of home appliances, the front shell of TV sets, especially large-screen color TVs, is one of the earliest and most widely used products with gas-assisted injection molding technology.
 

Basic principles of gas-assisted product and mold design

 1. When designing, first consider which wall thicknesses need to be hollowed out and which surface indentations need to be eliminated, and then consider how to connect these parts to become air channels.
 2. Large structural parts: thinning all over and thickening partially as an airway.
 3. The air channel should follow the main material flow direction and be balanced over the entire cavity while avoiding closed-circuit air channels.
 4. The cross-sectional shape of the air channel should be close to circular to allow smooth gas flow; the cross-sectional size of the air channel should be appropriate, too small may cause gas penetration, too large may cause fusion marks or air pockets.
 5. The airway should extend to the final filling area (generally on the non-appearance surface), but not to the edge of the cavity.
 6. The main airway should be as simple as possible, the branch airways should be as long as possible, and the ends of the branch airways can be gradually reduced to stop gas acceleration.
 7. The airway should be straight, not curved (the less curved the better), and the corners of the airway should be rounded with a larger radius.
 8. For multi-cavity molds, each cavity needs to be supplied by a separate gas nozzle.
 9. If possible, do not allow a second option for gas advancement.
 10. The gas should be confined to the airway and penetrate to the end of the airway.
 11. Precise cavity dimensions are very important.
 12. It is important that all parts of the product are cooled uniformly.
 13. When using gate feed, flow balance is important for uniform gas penetration.
 14. The accurate melt injection volume is very important, the error should not exceed 0.5% per injection.
 15. Overflow wells at the final filling can promote gas penetration, increase airway hollowing rate, eliminate hysteresis marks and stabilize product quality. The valve gate between the cavity and the overflow well can ensure that the final filling occurs in the overflow well.
 16. The small gate prevents the backflow of gas into the sprue when the air nozzle is fed.
 17. The inlet gate can be placed at a thin wall and at a distance of more than 30mm from the gas inlet to avoid gas penetration and backflow.
 18. The gas nozzle should be placed at the thick wall and located at the farthest distance from the last filling place.
 19. The direction of the air nozzle outlet should be the same as the direction of the material flow as far as possible.
 20. Keep the melt flow front at a balanced speed and avoid forming a V-shaped melt flow front. When using shortage injection, the volume of the unfilled cavity before the air inlet should be no more than half of the total volume of the air channel.
 21. When using full material injection, half of the total volume of the airway should be approximately equal to the volume shrinkage of the plastic in the cavity with reference to the pressure, specific volume, and temperature relationship of the plastic.